The invention relates to a device for converting calorific energy into mechanical energy, comprising at least one combustion chamber having connected thereto at least one inlet duct for fuel, at least one inlet duct for combustion air in which a first restriction is incorporated, and at least one outlet duct for flue gases. A flue-gas return duct is provided having an inlet and an outlet, the inlet being connected to the flue gas outlet duct and the outlet to the part of the combustion air inlet duct which is situated between the combustion chamber and the first restriction. The flue gas return duct incorporates a second restriction which is constructed such that the mass flow of flue gas which passes through in each operating condition of the device is at least substantially proportional to the root of the pressure difference prevailing across the second restriction.
A device of the kind set forth has been proposed in the U.S. Pat. No. 3,846,985 in the name of Applicant. Devices of this kind are, for example, hot-gas reciprocating engines, hot-gas turbines, internal combustion engines and the like. In these known devices a part of the flue gasses discharged from the device is branched off and, after mixing with combustion air, is returned to the combustion chamber. Because of their heat capacity the returned flue gases ensure that the combustion temperature in the combustion chamber does not become too high. It is thus achieved that nitrogen oxides are formed only to a limited extent. This is because the production of health-hazardous nitrogen oxides increases strongly as the temperature at which the combustion of the air-fuel mixture takes place is higher. Consequently, devices of this kind offer the advantage that air pollution is minimized.
By the use of a turbulent restriction in the flue-gas return duct and a laminar restriction in the combustion air inlet duct it is automatically achieved in the proposed device that when at small loads of the device, when comparatively small quantities of combustion air are supplied, comparatively large quantities of flue gas are recirculated, while in the case of large loads, when comparatively large quantities of combustion air are supplied to the device, comparatively small quantities of flue gas are recirculated.
The fact that a comparatively large quantity of flue gas is recirculated in the case of small loads is particularly attractive in hot-gas engines and internal combustion engines for traction purposes. These engines usually have a small load in city traffic, and air pollution should be minimized particularly in these circumstances.
In these circumstances a comparatively large recirculation of flue gases not only ensures that only small quantities of nitrogen oxides are produced, but also only a small quantity of carbon monoxide. Moreover, also the presence of non-combusted carbon hydrates and soot in the exhaust gases is prevented. The latter is so because the flue gases ensure proper mixing of air and fuel, which results in proper combustion. The fact that a comparatively small flue gas recirculation takes place in the case of large loads offers the advantage for the hot-gas engine and the hot-gas turbine that the combustion air fan, also drawing in flue gas, may be of a comparatively small power and small dimensions, while in the case of the internal combustion engine the maximum power to be delivered is only slightly reduced by the recirculation.
It was found that the favorable flue gas recirculation characteristic thus obtained, is disturbed if the part of the flue gas outlet duct which is situated beyond, viewed downstream, its connection with the flue gas return duct has an excessive flow resistance. This in fact concerns the actual exhaust pipe.
Because of the excessive flow resistance, the pressure which prevails at the area where the inlet of the flue-gas return is connected to the flue-gas outlet duct is not ambient pressure, but rather the ambient pressure increased by the pressure drop across the exhaust pipe. Because the flow through the exhaust pipe is normally turbulent and the pressure drop across the pipe, consequently, is proportional to the square of the mass flow of flue gas through this pipe, the pressure drop strongly increases in the case of larger loads. As a result, the mass flow of flue gas returned to the combustion chamber at these larger loads also increases. The favorable flue gas recirculation characteristic according to which comparatively little flue gas is returned in the case of comparatively large loads is thus lost. A reduced flow resistance of the exhaust pipe by a reduction of the pipe length is usually not feasible in vehicles, while an increased pipe diameter is undesirable in view of space and material cost price considerations.